Work to date has established that lymphocytes bear the same mutational burdens as myeloid stem cells. Age, chemotherapy and myelodysplasia are associated with above-average incidence of mutation at the reporter gene hprt (hypoxanthine ribosyltransferase). We plan to strengthen the data thus far compiled by increasing the normal cohort of 60 to 70 year olds, and by studying more patients with myelodysplasia. Approximately 10 more marrow and blood samples will be obtained on the CRC by October 1997.